http: drain in-flight per-request coroutines on shutdown (#74)

[EdmondDantes]

Fixes #74.

Server shutdown disposed server_scope while per-request handler coroutines were still suspended, aborting in debug builds (scope.c "Scope should be empty before disposal") and otherwise hanging the worker on the parked coroutines.

Fix

start() now drains server_scope on its lifecycle coroutine right after stop() wakes it (never inside stop(), which may be called from a handler living in server_scope), using the Scope's own orderly API:

1. awaitCompletion(Async\timeout(grace)) — let in-flight handlers finish within the setShutdownTimeout grace window (default 5s; 0 = cancel immediately).
2. cancel() + awaitAfterCancellation() — hard-terminate whatever is still parked and wait for it to settle.

A guarded fallback runs in http_server_free for the freed-mid-flight path (no-op in teardown/scheduler context).

server_scope is created with DISPOSE_SAFELY cleared so cancel() hard-terminates handlers instead of zombifying them — a dispose-safely scope would zombify them and awaitAfterCancellation would wait forever. Child request scopes inherit the non-safe mode.

Tests

• New tests/phpt/server/core/045-shutdown-drains-inflight.phpt.
• Full server phpt suite green (186: 46 core + 140) on debug; core 46/46 on release.
• Verified end-to-end on release with the issue's request_context() repro: 60s hang → 0.08s clean shutdown.

Note: the request_context()-in-handler variant of the repro also needs php-async 8f0983f (request_context first-accessor null fix); without it the handler fatals and the drain has nothing to wait on.

[github-actions]

Coverage

Total lines: 81.35% → 81.26% (-0.09 pp)

File	Baseline	Current	Δ	Touched
src/core/http_connection.c	68.03%	68.03%	+0.00 pp	●
src/core/http_connection.h	81.82%	81.82%	+0.00 pp	●
src/http2/http2_strategy.c	77.10%	77.10%	+0.00 pp	●
src/http3/http3_dispatch.c	79.45%	79.45%	+0.00 pp	●
src/http3/http3_listener.c	75.96%	76.16%	+0.20 pp
src/http_server_class.c	68.19%	67.71%	-0.48 pp	●